The invention relates to a process for producing fine monofilaments having improved abrasion resistance from polypropylene having a melt flow index (MFI) 230xc2x0 C./2.16 kg of 2-16 g/10 min, to a monofilament of polypropylene having a melt flow index (MFI) 230xc2x0 C./2.26 kg of 2-16 g/10 min having improved abrasion resistance and a linear density of 5-20 dtex (0.027 mm-0.053 mm) and also to the use thereof.
Industrial fabrics composed of polypropylene are becoming of increased interest in the automotive industry, in particular because they are lighter and more stable to environmental effects and mechanical stress than other thermoplastic materials. There is a particular demand for fine monofilaments, which permit a further weight reduction. By fine monofilaments are meant monofilaments having a linear density of less than 30 dtex and especially less than 25 dtex.
However, monofilaments composed of polypropylene only have the disadvantage of severe dusting in the weaving operation as a consequence of the low abrasion resistance of pure polypropylene, although other thermoplastics are known to have an abrasion problem too. For instance, EP-A2 0 784 107 mentions melt-spun polyamide, polyester and polypropylene monofils and shows that abrasion-resistant filaments are obtained with 70-99% by weight of fibre-forming polymer and 1-30% by weight of a maleic anhydride modified polyethylene-polypropylene rubber and further additives. However, the examples are limited to nylon 6 and polyethylene terephthalate and to a copolyamide of PA66 and PA6 as fibre-forming polymer. Spinning speeds are not reported. The relatively thick monofilaments exemplified are useful for papermachine wire fabrics and lawn mower wires. The production of relatively fine polypropylene monofilaments is not disclosed.
EP-A-1059370 discloses a method for the production of polypropylene multifilaments for textile purposes. The starting material used is a metallocene-catalysed isotactic polypropylene having a melt flow index of less than 25 g per 10 minutes in order that the desired shrinkage properties may be achieved. Low-shrinkage filaments are preferably produced using polypropylene chips having a high MFI value. The yarns produced are only described in general terms. Monofilaments are not described at all.
EP-A-0028844 describes a textile multifil polypropylene filament yarn. The starting polymer is a polypropylene having a melt flow index between about 20 and 60. The problem of abrasion encountered in the processing of fine monofilaments was evidently not observed under the reported spinning and stretching conditions and in the course of the further processing.
It is an object of the present invention to provide an economical process for producing fine abrasion-resistant monofilaments composed of polypropylene. It is a further object of the present invention to produce polypropylene fine monofilaments having improved resistance to abrasion in weaving.
It is yet a further object of the present invention to provide the use of a fine monofilament having good abrasion resistance for producing industrial fabrics.
These objects are achieved according to the invention when a compound consisting of 80 to 99.9% by weight of chips and 20 to 0.1% by weight of an additive is added to the extruder, the melt is spun at a speed of at least 1200 m/min, the fibre is cooled in an air bath at room temperature, supplementarily stretched at a temperature of 110 to 150xc2x0 C. to a linear density of 5-20 dtex (0.027 mm-0.053 mm) and wound up. It is essential here that the additive has been thoroughly dispersed in the polypropylene and that no difference is observable in the resulting monofilament.
This is the first time that it has been possible to produce fine polypropylene monofilaments using a spinning speed of 1200 m/min. It has been determined to be advantageous to use additives selected from modified polyolefins and aliphatic diesters.
Particularly advantageous additives are modified polyolefins used in an amount of 4.5 to 15% by weight, especially 6 to 13% by weight and preferably 8 to 12% by weight of polypropylene/polyethylene having a melting point  greater than 140xc2x0 C. A melting point of less than 140xc2x0 C. is inconvenient to meter. This is because at temperatures below 140xc2x0 C. the chips stick together in the extruder. Using less than 4.5% by weight and more than 15% by weight of polypropylene/polyethylene makes for a monofilament having an unsatisfactory abrasion resistance. This variant surprisingly requires no further additives to achieve outstanding abrasion resistance.
In a further variant, the additive used is advantageously 3-10% by weight, especially 3 to 7% by weight and preferably 3 to 6% by weight of an impact modifier. Useful impact modifiers do not soften at up to 100xc2x0 C. and are constructed of linear styrene-ethylene/butylene-styrene block copolymers or alloys of linear styrene-ethylene/butylene-styrene block copolymer//styrene-ethylene/butylene biblock.
In a further variant, the -additive used is advantageously 0.1-0.2% by weight of a plasticizer. Diisononyl adipate is a most suitable plasticizer.
In a further variant, the additive used is advantageously 0.05-1.0% by weight and especially 0.3 to 1.0% by weight of a lubricant. Useful lubricants are particularly metal salts of carboxylic acids, linear or branched hydrocarbons, fluoroelastomers, polydimethyl-siloxanes.
In a further variant, it is advantageous to use fillers as additive. Particularly useful fillers are 0.01-0.1% by weight of Aerosils and 0.1-1.0% by weight of calcium carbonate.
In a further variant, the additive is a compounded combination of 2-10% by weight of an impact modifier, 0.1-0.2% by weight of plasticizer, 0.01-0.1% by weight of Aerosil or 0.1-1.0% by weight of calcium carbonate as fillers, 0.05-1.0% by weight of lubricants and 0.1-0.5% by weight of heat stabilizers. Useful heat stabilizers include sterically hindered phenols, phosphites and phosphonites.
The main polymer contemplated for the monofilaments according to the invention is a polypropylene having a melt flow index (MFI) 230xc2x0 C./2.16 kg of 2-16 g/10 min and a linear density of 5-20 dtex (0.027 mm-0.053 mm). A melt flow index of less than 2 g/10 min has the disadvantage that the melt-spinning operation requires excessively high temperatures, which leads to destruction of the polymer. A melt flow index of more than 16 g/10 min has the disadvantage that the resulting abrasion resistance is inadequate. An abrasion resistance score xe2x89xa62 is achieved by a monofilament which is easily weavable into a textile fabric and produces a surprising cleanness.
The monofilament according to the invention has a tenacity of at least 47 cN/tex and an elongation at break of less than 45%.
The monofilament according to the invention has a mechanical constant (constante mxc3xa9chanique) of at least 285 cN/tex.
The invention will now be more particularly described by way of example.
The fibre-forming monofilament used was in all runs a polypropylene having a melt flow index (MFI) 230xc2x0 C./2.16 kg of 12.0 g/10 min. For each run, 5 kg of polypropylene chips are blended using tinplate cans and a tumble mixer. Three different blending methods were used, depending on the additive. The individual methods are described in the examples. The blend of chips and additive is directly introduced into the extruder and melted.
Extruder Diameter 38 mm:
Maximum p=100 bar
Throughput: 1-10 kg/h
6 heatable zones
Spin pack: Diphyl-heated; 1 spinning position
Spin pump: 3-27 rpm
Spinnerets: diameter outer/inner=85/70 mm
Quench chimney: 450-1100 m3/h; 1=1.3 m
Extruder temperatures for zones 1 to 5: 180/230/250/250/265/275xc2x0 C. 
Pack+spinnerets: 275/275xc2x0 C.
Throughput: 1.65 kg/h
Quench air: 700 m3/h
Melt temperature:=280xc2x0 C.
Spinning take-off speed: 1200 m/min
Stretching is carried out using a laboratory stretching range equipped with two stretching units each made up of a godet (Ø=10 cm) and a separating roller.
The monofilaments undergoing a stretching operation pass through the following elements:
Yarn brake
Stretching unit V1, equipped with an additional feed or rubber roll. No snubbing pins.
Hotplate 20 cm in length and positioned 20 cm away from the stretching unit
Stretching unit V2
Traveller ring spindle
The variants are stretched using a stretch ratio of 3.6:1 and a hotplate (20 cm) at 130xc2x0 C. The take-off speed of stretching unit V2 is 514 m/min.
In the case of the modified polyolefins, the chips blend, consisting of polypropylene and modified polyolefin, PP/PE melting point  greater than 140xc2x0 C., is mixed for one hour.
In the case of the modified polyolefins, the chips blend, consisting of polypropylene and impact modifier, is mixed for one hour. It is advantageous to add an antistat, such as 0.1% of Atmer 110 (trade mark of Uniqema) in the case of these blends.
The plasticizer is added to the polypropylene chips and mixed in for two hours.
In the case of the pulverulent additives such as fillers, lubricants, heat stabilizers, etc., the chips are first tumbled for half an hour with a coupling agent such as Basilon M100 (trade mark of Bayer AG) before the remaining additives are added and mixed in for a further one and half hours. This series of runs includes the incorporation of calcium carbonate into polypropylene similarly to the above description.
In this example, a lubricant is added to the polymer in various amounts.
Same preparation as in Example 5.
In the case of the additives in the form of a combination of different compounds, run 17 contains two different lubricants (0.2 and 0.05%) and Aerosil at 0.05%. Runs 18+19 are based on three additives.
0.35% of heat stabilizer, 0.3% of calcium carbonate and 0.15% of lubricant 4
0.5% of heat stabilizer, 0.2% of lubricant 4 and 0.01% of Aerosil
Same preparation as in example 5.
The results are summarized in Table 1.